1. CS 3700 Networks and Distributed Systems
Christo Wilson
8/22/2012
CS 3700 Networks and Distributed Systems
Network Layer
(Putting the Net in Internet)
Revised 9/21/16
Defense

2. Network Layer Application Session Transport Network Data Link Physical
Function:
Route packets end-to-end on a network, through multiple hops
Key challenge:
How to represent addresses
How to route packets
Scalability
Convergence
Application
Presentation
Session
Transport
Network
Data Link
Physical

3. Routers, Revisited How to connect multiple LANs?
LANs may be incompatible
Ethernet, Wifi, etc…
Connected networks form an internetwork
The Internet is the best known example
Routers

4. Structure of the Internet
Network 3
Network 1
Network 4
Network 2
Ad-hoc interconnection of networks
No organized topology
Vastly different technologies, link capacities
Packets travel end-to-end by hopping through networks
Routers “peer” (connect) different networks
Different packets may take different routes

5. Internetworking Issues
Internet Service Model
Best-effort (i.e. things may break)
Store-and-forward datagram network
Lowest common denominator
Naming / Addressing
How do you designate hosts?
Routing
Must be scalable (i.e. a switched Internet won’t work)
Service Model
What gets sent?
How fast will it go?
What happens if there are failures?
Must deal with heterogeneity
Remember, every network is different

6. Outline Addressing IPv4 Protocol Details IPv6 Class-based CIDR
Packed Header
Fragmentation
IPv6

7. Possible Addressing Schemes
Flat
e.g. each host is identified by a 48-bit MAC address
Router needs an entry for every host in the world
Too big
Too hard to maintain (hosts come and go all the time)
Hierarchy
Addresses broken down into segments
Each segment has a different level of specificity

8. Example: Telephone Numbers
3278
Very General
West Village H
Room 248
West Village G
Room 213
Updates are Local
Very Specific

9. Binary Hierarchy Example
1**
Datagram, Destination = 101
10*
100
101
11*
0**
110
00*
000
111
001
01*
010
011

10. IP Addressing IPv4: 32-bit addresses
Usually written in dotted notation, e.g
Each number is a byte
Stored in Big Endian order 8 16 24 31
Decimal
192
168 21 76
Hex C0 A8 15 4C
Binary

11. IP Addressing and Forwarding
Routing Table Requirements
For every possible IP, give the next hop
But for 32-bit addresses, 232 possibilities!
Too slow: 4 * 10GE ports needs 176Gbps memory bandwidth
DRAM is 1-6 Gbps, TCAM is faster but 400x more expensive than DRAM
Hierarchical address scheme
Separate the address into a network and a host 31
Pfx
Network
Host
Known by all routers
Known by edge (LAN) routers

12. Classes of IP Addresses
Christo Wilson
8/22/2012
Classes of IP Addresses 1 8 16 24 31
Example: MIT
18.*.*.*
Class A
Ntwk
Host
1-126 2 8 16 24 31
Example: NEU
*.*
Class B 10
Network
Host 3 8 16 24 31
Example: *
Class C
110
Network
Host
Defense

13. How Do You Get IPs? IP address ranges controlled by IANA
Internet Assigned Number Authority
Roots go back to 1972, ARPANET, UCLA
Today, part of ICANN
IANA may grant you a class of IPs
You may then begin installing routers that advertise routes to your new IP range

14. Subtree size determined by network class
Two Level Hierarchy
Pfx
Network
Host … …
Subtree size determined by network class

15. Class Sizes Way too big Too many network IDs Too small to be useful
Prefix Bits
Network Bits
Number of Classes
Hosts per Class A 1 7
27 – 2 = 126
(0 and 127 are reserved)
224 – 2 = 16,777,214
(All 0 and all 1 are reserved) B 2 14
214 = 16,398
216 – 2 = 65,534 C 3 21
221 = 2,097,512
28 – 2 = 254
Total: 2,114,036
Too many network IDs
Too small to be useful

16. Subnets Problem: need to break up large A and B classes
Solution: add another layer to the hierarchy
From the outside, appears to be a single network
Only 1 entry in routing tables
Internally, manage multiple subnetworks
Split the address range using a subnet mask
Subnet Mask is
Pfx
Ntwk
Subnet
Host
Subnet Mask:

17. Subnet Example Extract network: Extract subnetwork: Extract host:
10 means this is a class B network
Class B uses 16 bits total for the network name
Subnet Example
IP Address:
Extract network:
Subnet mask is
Subnet Mask:
&
Result:
IP Address:
&
Subnet Mask:
Result:
Extract subnetwork:
Extract host:
IP Address:
& ~( )
Subnet Mask:
Result:

18. N-Level Subnet Hierarchy
Pfx
Network
Subnet
Host …
Tree does not have a fixed depth
Increasingly specific subnet masks …
Subtree size determined by length of subnet mask …

19. Example Routing Table Question: 128.42.222.198 matches four rows
Address Prefix
Subnet Mask
Prefix After Masking (in Binary)
Next Hop
******** ******** ******** ********
Port 4
******** ******** ********
Port 2
******** ********
Port 3
******* ********
Port 5
********
Port 1
Question: matches four rows
Which router do we forward to?
Longest prefix matching
Use the row with the longest number of 1’s in the mask
This is the most specific match

20. Subnetting Revisited
Question: does subnetting solve all the problems of class-based routing? NO
Classes are still too coarse
Class A can be subnetted, but only 126 available
Class C is too small
Class B is nice, but there are only 16,398 available
Routing tables are still too big
2.1 million entries per router

21. Classless Inter Domain Routing
CIDR, pronounced ‘cider’
Key ideas:
Get rid of IP classes
Use bitmasks for all levels of routing
Aggregation to minimize FIB (Forwarding Information Base)
Arbitrary split between network and host
Specified as a bitmask or prefix length
Example: Northeastern
with netmask
/ 16

22. Example CIDR Routing Table
Prefix
Netmask
Prefix After Masking (in Binary)
IP Address Range 19
***** ********
– 31.*
***** ********
– 63.*
***** ********
– 95.* 18
****** ********
– 191.*
****** ********
– 255.*
Hole in the Routing Table: No coverage for 96 – 127
Missing entry: /19

23. CIDR Aggregation Example
Christo Wilson
8/22/2012
CIDR Aggregation Example
Same length netmask
All bits match except for the last one
Same port
Prefix
Netmask
Prefix After Masking (in Binary)
Next Hop 19
***** ********
Port 1
***** ******** 18
****** ********
****** ********
Port 2
****** ********
Port 3 17 18
****** ********
******* ********
Port 1
Port 1
Aggregation allows multiple routes to be compressed together to shrink the size of the routing table
Defense

24. Size of CIDR Routing Tables
From
CIDR has kept IP routing table sizes in check
Currently ~450,000 entries for a complete IP routing table
Only required by backbone routers

25. Takeaways Hierarchical addressing is critical for scalability
Not all routers need all information
Limited number of routers need to know about changes
Non-uniform hierarchy useful for heterogeneous networks
Class-based addressing is too course
CIDR improves scalability and granularity
Implementation challenges
Longest prefix matching is more difficult than schemes with no ambiguity

26. Outline Addressing IPv4 Protocol Details IPv6 Class-based CIDR
Packed Header
Fragmentation
IPv6

27. IP Datagrams IP Datagrams are like a letter Totally self-contained
Include all necessary addressing information
No advanced setup of connections or circuits 4 8 12 16 19 24 31
Version
HLen
DSCP/ECN
Datagram Length
Identifier
Flags
Offset
TTL
Protocol
Checksum
Source IP Address
Destination IP Address
Options (if any, usually not)
Data

28. IP Header Fields: Word 1 Version: 4 for IPv4
Header Length: Number of 32-bit words (usually 5)
Type of Service: Priority information (unused)
Datagram Length: Length of header + data in bytes 4 8 12 16 19 24 31
Version
HLen
DSCP/ECN
Datagram Length
Identifier
Flags
Offset
TTL
Protocol
Checksum
Limits packets to 65,535 bytes
Source IP Address
Destination IP Address
Options (if any, usually not)
Data

29. IP Header Fields: Word 3 Used to implement trace route
Time to Live: decremented by each router
Used to kill looping packets
Protocol: ID of encapsulated protocol
6 = TCP, 17 = UDP
Checksum 4 8 12 16 19 24 31
Version
HLen
DSCP/ECN
Datagram Length
Identifier
Flags
Offset
TTL
Protocol
Checksum
Source IP Address
Used to implement trace route
Destination IP Address
Options (if any, usually not)
Data

30. IP Header Fields: Word 4 and 5
Source and destination address
In theory, must be globally unique
In practice, this is often violated 4 8 12 16 19 24 31
Version
HLen
DSCP/ECN
Datagram Length
Identifier
Flags
Offset
TTL
Protocol
Checksum
Source IP Address
Destination IP Address
Options (if any, usually not)
Data

31. Problem: Fragmentation
MTU = 4000
MTU = 2000
MTU = 1500
Datagram
Dgram1
Dgram2 1 2 3 4
Problem: each network has its own MTU
DARPA principles: networks allowed to be heterogeneous
Minimum MTU may not be known for a given path
IP Solution: fragmentation
Split datagrams into pieces when MTU is reduced
Reassemble original datagram at the receiver

32. IP Header Fields: Word 2
Identifier: a unique number for the original datagram
Flags: M flag, i.e. this is the last fragment
Offset: byte position of the first byte in the fragment
Divided by 8 4 8 12 16 19 24 31
Version
HLen
TOS
Datagram Length
Identifier
Flags
Offset
TTL
Protocol
Checksum
Source IP Address
Destination IP Address
Options (if any, usually not)
Data

33. Fragmentation Example
MTU = 4000
MTU = 2000
MTU = 1500
Length = 2000, M = 1
Offset = 0 IP
Data
Length = 3820, M = 0 20
1980
IP Hdr
Data
1980
+ 1820
= 3800 20
3800
Length = 1840, M = 0
Offset = 1980 IP
Data 20
1820

34. Fragmentation Example
MTU = 2000
MTU = 1500
Length = 2000, M = 1
Offset = 0
Length = 1500, M = 1
Offset = 0 IP
Data IP
Data
Length = 1500, M = 1
Offset = 1980 20
1980 20
1480 IP
Data
Length = 520, M = 1
Offset = 1480
1480
+ 500
= 1980 20
1480
Length = 1840, M = 0
Offset = 1980
Length = 360, M = 0
Offset = 3460 IP
Data IP
Data 20
500 IP
Data 20
1820 20
340

35. IP Fragment Reassembly
Length = 1500, M = 1, Offset = 0
Performed at destination
M = 0 fragment gives us total data size
360 – = 3800
Challenges:
Out-of-order fragments
Duplicate fragments
Missing fragments
Overlapping fragments
Basically, memory management nightmare
Data IP 20
1480
Length = 520, M = 1, Offset = 1480
Data IP 20
500
Length = 1500, M = 1, Offset = 1980
Data IP 20
1480
Length = 360, M = 0, Offset = 3460
Data IP 20
340

36. Fragmentation Concepts
Highlights many key Internet characteristics
Decentralized and heterogeneous
Each network may choose its own MTU
Connectionless datagram protocol
Each fragment contains full routing information
Fragments can travel independently, on different paths
Best effort network
Routers/receiver may silently drop fragments
No requirement to alert the sender
Most work is done at the endpoints
i.e. reassembly

37. Fragmentation in Reality
Fragmentation is expensive
Memory and CPU overhead for datagram reconstruction
Want to avoid fragmentation if possible
MTU discovery protocol
Send a packet with “don’t fragment” bit set
Keep decreasing message length until one arrives
May get “can’t fragment” error from a router, which will explicitly state the supported MTU
Router handling of fragments
Fast, specialized hardware handles the common case
Dedicated, general purpose CPU just for handling fragments

38. Outline Addressing IPv4 Protocol Details IPv6 Class-based CIDR
Packed Header
Fragmentation
IPv6

39. The IPv4 Address Space Crisis
Problem: the IPv4 address space is too small
232 = 4,294,967,296 possible addresses
Less than one IP per person
Parts of the world have already run out of addresses
IANA assigned the last /8 block of addresses in 2011
Region
Regional Internet Registry (RIR)
Exhaustion Date
Asia/Pacific
APNIC
April 19, 2011
Europe/Middle East
RIPE
September 14, 2012
South America
LACNIC
June 10, 2014
North America
ARIN
September 22, 2015
Africa
AFRINIC
January 2022 (Projected)

40. IPv6 IPv6, first introduced in 1998 Address format 128-bit addresses
4.8 * 1028 addresses per person
Address format
8 groups of 16-bit values, separated by ‘:’
Leading zeroes in each group may be omitted
Groups of zeroes can be omitted using ‘::’
2001:0db8:0000:0000:0000:ff00:0042:8329
2001:0db8:0:0:0:ff00:42:8329
2001:0db8::ff00:42:8329

41. IPv6 Trivia Who knows the IP for localhost? What is localhost in IPv6?
What is localhost in IPv6?
::1

42. IPv6 Header Double the size of IPv4 (320 bits vs. 160 bits)4 8 12 16 19 24 31
Version
DSCP/ECN
Flow Label
Datagram Length
Next Header
Hop Limit
Version = 6
Same as IPv4
Groups packets into flows, used for QoS
Source IP Address
Same as IPv4
Same as Protocol in IPv4
Same as TTL in IPv4
Destination IP Address

43. Differences from IPv4 Header
Several header fields are missing in IPv6
Header length – rolled into Next Header field
Checksum – was useless, so why keep it
Identifier, Flags, Offset
IPv6 routers do not support fragmentation
Hosts are expected to use path MTU discovery
Reflects changing Internet priorities
Today’s networks are more homogeneous
Instead, routing cost and complexity dominate
No security vulnerabilities due to IP fragments

44. Performance Improvements
No checksums to verify
No need for routers to handle fragmentation
Simplified routing table design
Address space is huge
No need for CIDR
Standard subnet size is 264 addresses
Simplified auto-configuration
Neighbor Discovery Protocol
Used by hosts to determine network ID
Host ID can be random!

45. Deployment Challenges
HTTP, FTP, SMTP, RTP, IMAP, …
TCP, UDP, ICMP
IPv4
Ethernet, x, DOCSIS, …
Fiber, Coax, Twisted Pair, Radio, …
Switching to IPv6 is a whole-Internet upgrade
All routers, all hosts
ICMPv6, DHCPv6, DNSv6
June 2012: 0.2% of global traffic was IPv6